1. Field of the Invention
The invention relates to a wiring unit that has a belt-like sheet substrate and a plurality of feeding terminals formed on the elongated sheet substrate. In addition, the invention further relates to a fluid discharging head such as a liquid drop discharging head that includes a wiring unit having such a configuration and a piezoelectric unit having a plurality of driving electrodes formed on one surface of the piezoelectric layer thereof.
2. Description of the Related Art
As an example of various kinds of liquid drop discharging heads such as an ink ejection head that is mounted on an ink jet printing apparatus, a head device that has a fluid channel unit, a piezoelectric unit, and a wiring unit is known in the technical field to which the present invention pertains. For example, a known head device, e.g., the device described in Japanese Unexamined Patent Application Publication No. 2007-90627, has a fluid channel unit having a plurality of inner flow passages, each of which is in fluid communication with the corresponding one of nozzle holes that are formed in the lower surface of the head, a piezoelectric unit that is attached to the upper surface of the fluid channel unit, and a wiring unit that is attached to the upper surface of the piezoelectric unit with an electro-conductive material being provided between the piezoelectric unit and the wiring unit.
A plurality of pressure chamber holes, e.g., pressure compartment cavities, is formed in a plurality of lines in the uppermost one of a plurality of layers that makes up the fluid channel unit. A plurality of driving electrodes is formed in a plurality of lines on the uppermost one of a plurality of layers that makes up the piezoelectric unit so as to correspond to the layout of the plurality of pressure chambers. The wiring unit has a substantially belt-like sheet substrate, which is a base substrate material, as well as a plurality of conducting wires formed on the sheet substrate and a plurality of feeding terminals formed on the sheet substrate. The plurality of feeding terminals is arrayed in a plurality of lines on one end area portion of the elongated sheet substrate. These lines of feeding terminals are formed at positions opposite the lines of driving electrodes. A driver is provided on the other end area portion of the elongated sheet substrate. The driver outputs electric signals to the driving electrodes through the conducting wires and the feeding terminals so as to drive the piezoelectric unit.
When a liquid drop discharging head is in operation, heat is generated, mostly at a driver. The heat that has been generated at the driver is communicated to feeding terminals through conducting wires. The heat that has been transmitted to the feeding terminals is further communicated to a piezoelectric unit and then to ink that flows inside a fluid channel unit. In the operation of a known liquid drop discharging head, heat that has been generated at the driver is communicated to the feeding terminals that are formed in lines on the one end area portion of the elongated sheet substrate through the conducting wires. Accordingly, the amount of heat that is communicated to some feeding terminals that are provided at positions that are relatively close to the driver is not the same as the amount of heat that is communicated to other feeding terminals that are provided at positions that are relatively remote from the driver.
Such a difference in heat amount causes lack of uniformity in the distribution of heat in the piezoelectric unit and the fluid channel unit, which might result in, for example, variation in the operation characteristics of the piezoelectric layer of the piezoelectric unit, or variation in liquid drop discharging characteristics of nozzle holes that is attributable to lack of uniformity in the viscosity of fluid that flows through flow passages in the fluid channel unit.
A known wiring unit that is provided with a metal plate having high thermoelectric power, e.g., a heat equalization plate, which is bonded onto an area at which feeding terminals are formed, has been proposed in the art in an effort to improve heat distribution uniformity. However, if such a heat equalization plate is additionally provided, it increases the number of parts and the number of bonding steps in the production of a liquid drop discharging head. As another disadvantage, the additional heat equalization plate makes the weight of a liquid drop discharging head heavier. In addition, a larger motor is necessary for driving a liquid drop discharging head.
In order to meet an increasing demand for high speed printing, in the field of an ink jet printer, the processing speed of a driver is getting faster and faster with the number of nozzle holes getting larger and larger. In addition, in order to meet an increasing demand for a compact ink ejection head, the array density of nozzle holes is getting greater and greater. As the processing speed of a driver increases, so does the amount of heat generated at the driver. For this reason, the importance of overcoming the problem of the lack of heat distribution uniformity explained above also increases. In addition, as the number of nozzle holes and the array density thereof increases, which involves an increase in the number of feeding terminals and the array density thereof, the number of conducting wires that provide electric connection between the driver and the feeding terminals also increases.
As the array density of the feeding terminals increases, a terminal-to-terminal gap, that is, an inter-feeding-terminal distance between two feeding terminals arrayed adjacent to each other decreases.